1. Field of Invention
The present invention relates to hearth rolls for a heating furnace and a soaking furnace of a vertical heat treating furnace. The present invention also relates to a vertical heat treating furnace using the hearth rolls.
2. Description of Related Art
Vertical heat treating furnaces are ordinarily divided into respective sections of a heating furnace, a soaking furnace, and a cooling furnace, and a predetermined heat treatment cycle is performed. Hereinafter, the heating furnace and the soaking furnace contained in the vertical heat treating furnace are described as one set of equipment in the present invention, and are referred to as "a heating/soaking furnace." Further, the vertical heat treating furnace includes a plurality of hearth rolls as transfer rolls located on the upper portion and the lower portion of the vertical heat treating furnace, and a metal strip is passed while being suspended by these hearth rolls and subjected to a necessary heat treatment in the process.
However, because metal strips to be passed are not always flat and include a bent portion and a locally extended portion, a transfer problem such as meandering and the like is liable to occur while they are being passed.
In particular, large vertical heat treating furnaces (of such class in which the distance between upper rollers and lower rollers exceeds 15-20 m) have been constructed in large numbers, and the prevention of transfer problems is a leading problem to be solved in these furnaces. To prevent problems in the transfer of metal strip, the shape of hearth rolls has been variously devised such as by the formation of a crown and the like on the hearth rolls. However, when a large crown is formed to prevent meandering as a transfer problem, there is a possibility that a problem called "buckling", in which a metal strip is buckled in a width direction, can occur. This problem is significant, particularly when the furnace temperature is high and a problem is caused when a sheet is passed. Thus,, buckling is one of leading causes of lowered operating efficiency of equipment and product yield.
There have been made various devices to effectively prevent meandering and buckling, which are problems caused when a metal strip is passed in a vertical heat treating furnace.
Japanese Unexamined Patent Application Publications Nos. 55-100919 and 57-137431, for example, disclose controlling the crown of a roll using the thermal expansion in a hearth roll by devising the inner structure of the hearth roll.
Further, Japanese Unexamined Patent Application Publications Nos. 7-331335 and 3-47926 disclose controlling the crown of a hearth roll by controlling its temperature by applying heat to the hearth role from the outside.
Japanese Unexamined Patent Application Publications Nos. 8-199247, 7-138656, 58-20739, and 52-136812 disclose conventional examples in which the shape of a hearth roll itself is devised. These publications disclose a hearth roll having a one-stepped taper, which is arranged such that the central portion of the hearth roll has a flat shape or a crown shape, with both sides of the roll having a taper.
The applicant has disclosed in Japanese Unexamined Patent Application Publication No. 59-116331 that a roll having a two-stepped taper shape can be used together with the above roll having a one-stopped taper shape and a crown shape.
Recently, however, a steel sheet having width much larger than that of a conventional steel sheet has been required as a steel sheet for integrally forming an automobile body.
Therefore, it has been required to pass a metal sheet, in particular, a steel sheet having a wider range of sheet width, which is larger than a conventional range, in a single vertical heat treating furnace.
Regarding a steel sheet for automobiles, a line was conventionally operated with a sheet width ranging from 800 mm to 1500 mm. Recently, it has been required to pass a steel sheet having a sheet width of about 800-1500 mm, and sometimes a steel sheet having a sheet width larger than this, in the same line.
When a range of sheet width is wide as described above, a transfer problem cannot be sufficiently overcome by simply devising only the roll shape of a one-stepped taper roll as in the conventional technology.
An optimum roll shape has been known as to the one-stepped taper roll and used as an effective means for preventing meandering and buckling in the operation in the conventional range of sheet width. However, the roll shape cannot be used as it is in a wide range of sheet widths having a ratio of maximum to minimum sheet width of, for example, 2 or more.
The present inventors have discovered the one-stepped taper roll has a problem in that while it can effectively prevent the occurrence of buckling in a wide metal strip when the inclination of a taper is reduced, meandering is liable to occur in a narrow metal strip. In contrast, when the inclination of the taper is increased, while meandering can be effectively prevented in a narrow metal strip, buckling is liable to occur in a wide metal strip, particularly when its thickness is thin. Thus, it is impossible to follow a wide range of sheet width and to cope with a problem caused by the wide range of sheet width by the use of the one-stepped taper roll.
Further, even if the method of controlling the crown of a hearth roll by temperature control is applied, it is impossible to follow the wide range of sheet thickness and to cope with a problem caused by the wide range using this method, and it is necessary to reconfigure equipment on a large scale to follow the wide range of sheet width.
Further, a metal strip can be passed stably in a vertical heat treating furnace even by a conventional hearth roll to a certain extent when the metal strip is in a steady state in which it is passed at an approximately constant speed. However, when operating conditions are varied in a furnace to treat metal strips having a wide variety of sizes and various kinds of metal strips, the sheet passing speed is often changed considerably. Meandering and buckling often occur when the speed is changed (by changes corresponding to 40-50% of a steady speed). In the conventional hearth roll, it is very difficult to achieve a stable sheet passing property by taking even the change of sheet passing speed into consideration, and further it is not easy to achieve this property even by the use of the two-stepped taper roll.
In a continuous annealing furnace for steel strip, for example, an ordinary sheet passing speed is about 200-400 m/min in a steady state.